java – location of books’position in shelf which are inside a row

I’m working on my homework on library management system.
among class that I’ve created , I’ve got class book and library. the issue is , I don’t know what attributes to use for my library class. the homework says like this =>

the library has got rows (by the way 4). each row has got shelves which are numbered from 1 to n . And shelves are divided in position (or places). for each shelf , the number of places isn’t the same.

I’a beginner in learning java. and I need your help

I’ve made this little bit code in order to see my post accepted , because they say my post must contain at least 3 lines of code . So don’t give it a clue. thanks

public class generateRandom{ 
    public static void main(String args()) 
        // create instance of Random class 
        Random rand = new Random(); 
        // Generate random integers in range 0 to 999 
        int rand_int1 = rand.nextInt(1000); 
        int rand_int2 = rand.nextInt(1000); 
        // Print random integers 
        System.out.println("Random Integers: "+rand_int1); 
        System.out.println("Random Integers: "+rand_int2); 
        // Generate Random doubles 
        double rand_dub1 = rand.nextDouble(); 
        double rand_dub2 = rand.nextDouble(); 
        // Print random doubles 
        System.out.println("Random Doubles: "+rand_dub1); 
        System.out.println("Random Doubles: "+rand_dub2);